1. Field of the Invention
This invention relates to optically pumped lasers and in particular but not exclusively to lasers which produce a substantially single transverse mode output.
2. Related Art
Diffraction limited, monomode output, optically pumped lasers, which use a simple, single pump source arrangement and are capable of producing monomode outputs around 100 mW, are useful for small-signal or low power applications. However, such lasers are not easily adapted to produce higher monomode powers, in the order of Watts.
Optically pumped lasers capable of producing higher power monomode outputs, in the order of Watts, have typically required complex combinations of more than one pump source to provide high pump powers. Methods of combining pump sources have included: multiplexing several single transverse mode diode lasers of different wavelengths; polarisation multiplexing two or more such diodes into a fibre cavity, the fibre being pumped from both ends of the cavity; and, the use of multiple pumping ports along the length of a fibre laser cavity.
A possible single optical pump source for a high power laser which can produce optical output powers in the order of Watts is a diode-laser array. Diode-laser arrays can be fabricated with high electrical to optical efficiencies and are very reliable, however, they suffer from two major drawbacks: the lasers in the array are substantially mutually incoherent, which means the ability to focus the output is poor; the geometry of the array is generally a poor match to the application with which it would be used.
An optical coupling device for manipulating the outputs from a high power, linear diode-laser array, to form a two-dimensional, symmetric virtual source, to end-pump a Nd:YAG laser, is described in a paper "Geometrical Transformation of Linear Diode-Laser Arrays for Longitudinal Pumping of Solid-State Lasers", IEEE Journal of Quantum Electronics, Vol. 28, No. 4, April 1992. In the device, two planes of surface relief binary optics were implemented to transform the linear array outputs into a 2-D spot source with radiance approaching the maximum possible according to the Radiance Theorem, and symmetrized shape and divergence.
An alternative method of converting the output of a diode-laser array to a focused spot uses a computer generated hologram (CGH).
To enable maximum launch of optical power from a high power pump source such as a diode-laser array into a laser cavity, even if using the above-mentioned coupling methods, it is preferable for the cavity to have a large numerical aperture (NA). Such a large NA provides that the cavity will probably be multimode at the pump source wavelength.
Although an LDA is the preferred pump source in the present invention, any other source, which is capable of supplying the necessary, high power, optical output, may be used.
One form of waveguide arrangement which is able to use a multimode pump source to produce a substantially monomode output is the `Polaroid` type optical fibre, available from the Polaroid Corporation.
This arrangement consists of an elliptical outer core, which is undoped, into which the pump input is launched, and an inner, circular, doped core. However, this arrangement has an extremely complex structure, which makes fabrication very difficult. Also, performance is highly dependent on launch conditions of energy into the fibre, which are complicated, again, by the shape of the outer core.